The reproductive ecology of the Pepperbark tree (Warburgia salutaris) in the Kruger National Park, South Africa
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Date
2021
Authors
Van den Bosch, Kaylee
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Abstract
Warburgia salutaris, the pepperbark tree, is an endangered tree that is greatly valued in
South Africa for its use in traditional medicine. Over-exploitation for medicinal purposes, habitat
degradation and loss have all contributed to the species being listed as IUCN Endangered. In the
Kruger National Park (KNP), W. salutaris fruit production is low and only produced on a few trees
and there is a very low proportion of juveniles, suggesting that recruitment is extremely limited in
this population. Therefore, in this thesis, the reproductive ecology of the KNP population is studied
and compared with a reproductively successful orchard population in Eston (KwaZulu-Natal), to
investigate factors that may contribute to limited sexual output in the former.
The aim of the first part of the study was to understand the pollination biology of W.
salutaris. Potential pollinators were observed and collected for further analysis; nectar volume and
sugar concentration were measured; five pollination treatments – autonomous self-,
geitonogamous self-, cross-, natural- and natural-bagged pollination (i.e. naturally pollinated
flowers were bagged to protect from seed predation) – were set-up to elucidate the breeding
system; pollen viability was assessed, and pollen tube growth was compared between pollination
treatments. The aim of the second part of the study was to understand the seed ecology of W.
salutaris. Immature fruit were bagged to assess fruit development in the KNP population and Eston
orchard; fruit were analysed to assess the effect of pre-dispersal seed predation and identify seed
predators; frugivore activity was observed in Leshiba Game and Nature Reserve (GNR), Limpopo,
to identify seed dispersers and assess the mode of dispersal; and seedling emergence trials were
conducted to compare seed germination between the natural KNP and Leshiba GNR populations
and the Eston orchard.
Results showed that W. salutaris flowers are visited by a variety of insects and Lepidoptera
(butterflies and moths) are the most likely effective pollinators. Average nectar volume and
sucrose concentration was 2.43 l and 39.74%, respectively. Warburgia salutaris is likely self incompatible as self-pollinated flowers in neither KNP nor Eston resulted in fully developed fruit.
In KNP, zero mature fruit were produced from either self- or cross-pollination; however, a small
percentage of self (5%) and cross (2%) pollinated flowers were fertilised and then aborted early in
development. Similarly, 94% of flowers that began to form fruit in the natural-bagged treatment
were aborted. Only two trees in one KNP sub-population produced and sustained fully developed
fruit through natural pollination in 2019. In 2020, the same two trees and an additional untagged
v
tree successfully developed fruit. Pollen viability was high (66–93%) across the five KNP sub populations and Eston, and percentage pollen tube occurrence for self- and cross-pollination was
higher in Eston (self: 60%, cross: 100%) than KNP (self: 33%, cross: 33%), and both self and cross
pollen tubes reached the ovules.
Bagged immature fruit were aborted early in development in the KNP population and Eston
orchard, and in KNP successfully developed fruit were only present on three of 10 trees in one
sub-population. Ceratitis cosyra, the marula fly, attacks W. salutaris fruit in KNP, Eston, and
Leshiba GNR. However, 80% of collected KNP fruit were unaffected by seed predation. Seeds
from mature fruit with immature larvae had a far higher germination percentage (77%) than seeds
in fruit with fully developed larvae (6%). Therefore, there is a short period where seeds remain
viable while infested with C. cosyra. Frugivore observations in Leshiba GNR revealed that
baboons and vervet monkeys eat W. salutaris fruit (except the exocarp) and ingest the seeds.
However, their role as effective dispersers of W. salutaris is not certain. Frugivore activity was not
observed in KNP, but evidence of frugivore activity (i.e. empty fruit exocarps) was found. In the
seedling emergence trials KNP had the highest percentage germination (61%), followed by Eston
(47%), and Leshiba (33%), and all ungerminated seeds were nonviable.
This study has expanded our knowledge of the reproductive ecology of W. salutaris and
confirms that sexual reproduction of the species in KNP results in very low fruit and seed
production. However, W. salutaris has many floral visitors, pollen viability is high, and seeds
that are produced germinate successfully. Therefore, the impaired reproductive output in KNP
appears to be in the high occurrence of aborted fruit, which was found for both self- and cross
hand-pollinations and natural-bagged pollination treatments. For self-pollination this may be
late-acting self-incompatibility, i.e. the ovule is aborted after it has been fertilised by self-pollen.
However, the even lower success of cross-pollination may be explained by a genetically linked
post-fertilisation failure. This is further supported by the higher percentage of pollen tube
occurrence compared to fruit production for both self and cross hand-pollinations. The already
limited fruit production in KNP is impacted further by pre-dispersal seed predation, lowering the
number of seeds that can eventually contribute to seedling recruitment. However, germination of
the desiccation-sensitive seeds and seedling establishment may only be possible in seasons with
fairly regular and high rainfall. Therefore, it is possible that W. salutaris relies on clonal
reproduction and resprouting in response to damage to persist in northern KNP. However, long term reliance on clonal reproduction may impair the ability to reproduce sexually, which will
ultimately be necessary to establish genetically diverse genets that have the potential to adapt to
future environmental changes.
Description
A dissertation submitted in fulfilment of the requirements for the degree Master of Science to the Faculty of Science, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg, 2021